This invention relates generally to polymeric materials and to methods of making and using such materials. The polymeric materials are hydrophilic, biocompatible, and suitable for use with biosensors, such as glucose sensors.
Biosensors are small devices that use biological recognition properties for selective detection of various analytes or biomolecules. Typically, the sensor will produce a signal that is quantitatively related to the concentration of the analyte. To achieve a quantitative signal, a recognition molecule or combination of molecules is often immobilized at a suitable transducer, which converts the biological recognition event into a quantitative response.
The need for the continuous monitoring of biological markers (analytes) in medicine has sparked a tremendous interest in the study of biosensors in recent years. Without question, the greatest interest has been geared toward the development of sensors to detect glucose. In particular, enzymatic (amperometric) glucose electrodes have been studied in more detail than any other biosensors. Electroenzymatic biosensors use enzymes to convert a concentration of analyte to an electrical signal. Immunological biosensors rely on molecular recognition of an analyte by, for example, antibodies. Chemoreceptor biosensors use chemoreceptor arrays such as those of the olfactory system or nerve fibers from the antennules of the blue crab Callinectes sapidus to detect the presence of amino acids in concentrations as low as 109 M. For a review of some of the operating principles of biosensors, see Bergveld, et al., Advances in Biosensors, Supplement 1, p. 31-91, Turner ed., and Collison, et al., Anal. Chem. 62:425-437 (1990).
Regardless of the type of biosensor, each will possess certain properties to function in vivo and provide an adequate signal. First, the elements of the biosensor should be compatible with the tissue to which it is attached, and be adequately safe such that allergic or toxic effects ate not exerted. Further, the sensor should be shielded from the environment to control drift in the generated signal. Finally, the sensor should accurately measure the analyte in the presence of proteins, electrolytes and medications, which may have the potential to interfere.
The biosensor of interest is an amperometric glucose sensor. There are several reasons for the wide-ranging interest in glucose sensors. In the healthcare arena, enzymatic glucose test strips ate useful for monitoring the blood sugar of patients with diabetes mellitus. A sensor that has the ability to continuously monitor the blood, or interstitial glucose of a person with diabetes could provide great insight into the level of control that they have over their disease and avoid the need for repeated blood draws. Additionally, a continuously monitoring glucose sensor is one of the critical components necessary for the development of an artificial pancreas. To make such a system possible, a reliable glucose sensor must communicate with an insulin pump.
An additional commercial application of this technology focuses on sensors that can be used to monitor fermentation reactions in the biotechnology industry. From a scientific and commercial standpoint, interest has grown beyond glucose to other analytes for the diagnosis of numerous medical conditions other than diabetes.
Amperometric glucose sensors and oxido-reductase enzymes that use O2 as a co-substrate, and are designed for subcutaneous or intravenous use, typically require both an outer membrane and an anti-interference membrane. The necessity for two distinct membranes is largely due to the fundamental nature of the sensor, as well as the environment in which the measurement is made.
A glucose sensor works by a reaction in which glucose reacts with oxygen in the presence of glucose oxidase (GOd) to form gluconolactone and hydrogen peroxide. The gluconolactone further reacts with water to hydrolyze the lactone ring and produce gluconic acid. The H2O2 formed is electrochemically oxidized at an electrode as shown below (Equation 1):
H2O2xe2x86x92O2+2exe2x88x92+2H+xe2x80x83xe2x80x83(I) 
The current measured by the sensor/potentiostat (+0.5 to +0.7 v oxidation at Pt black electrode) is the result of the two electrons generated by the oxidation of the H2O2. Alternatively, one can measure the decrease in the oxygen by amperometric measurement 0.5 to xe2x88x921 V reduction at a Pt black electrode).
The stoichiometry of the GOd reaction points to a challenge of developing a reliable glucose sensor. If oxygen and glucose are present in equimolar concentrations, then the H2O2 is stoichiometrically related to the amount of glucose that reacts at the enzyme. In this case, the ultimate current is also proportional to the amount of glucose that reacts with the enzyme. If there is insufficient oxygen for all of the glucose to react with the enzyme, then the current will be proportional to the oxygen concentration, not the glucose concentration. For the sensor to be a true glucose sensor, glucose must be the limiting reagent, i.e. the O2 concentration must be in excess for all potential glucose concentrations. For example, the glucose concentration in the body of a diabetic patient can vary from 2 to 30 mM (millimoles per liter or 36 to 540 mg/dl), whereas the typical oxygen concentration in the tissue is 0.02 to 0.2 mM (see, Fisher, et al., Biomed. Biochem. Acta. 48:965-971 (1989). This ratio in the body means that the sensor would be running in the Michaelis Menten limited regime and would be very insensitive to small changes in the glucose concentration. This problem has been called the xe2x80x9coxygen deficit problemxe2x80x9d. Accordingly, a method or system must be devised to either increase the O2 in the GOd enzyme layer, decrease the glucose concentration, or devise a sensor that does not use O2.
There is a need for a glucose sensor having a biocompatible membrane with an improved ratio of its oxygen permeability to it glucose permeability, and that offers physical and biological stability and strength, adhesion to the substrate, processibility (i.e. solubility in common organic solvents for the development of coatings from polymer lacquer and the ability to cut using laser ablation or other large scale processing method), the ability to be synthesized and manufactured in reasonable quantities and at reasonable prices, and compatibility with the enzyme as deposited on the sensor. The present invention fulfills these needs and provides other related advantages.
The invention provides a biocompatible membrane comprising a hydrophilic polyurea composition. The hydrophilic polyurea composition comprises the product of a reaction mixture comprising (a) an amino terminated polysiloxane, (b) a hydrophilic polymer selected from the group consisting of a diamino terminated copolymer of polypropylene glycol and polyethylene glycol, polyethylene glycol, polypropylene glycol and diamino polyethylene glycol having an average molecular weight (MD) of from about 400 to about 2000, and (c) a diisocyanate selected from the group consisting of hexamethylene-1,6-diisocyanate, dicyclohexylmethane 4,4xe2x80x2-diisocyanate, and isophorone diisocyanate, and constituting about 50 mole % of the reaction mixture. In this mixture, (a) and (b) constitute a polymeric portion of the reaction mixture, and when the mixture is reacted with (c), the end product polymer has a ratio of its diffusion coefficient for oxygen to its diffusion coefficient for glucose of from about 2,000 to about 10,000. In a preferred embodiment, the hydrophilic polyurea composition has a ratio of its diffusion coefficient for oxygen to its diffusion coefficient for glucose of from about 3,000 to about 7,000. In a more preferred embodiment, the hydrophilic polyurea composition has a ratio of its diffusion coefficient for oxygen to its diffusion coefficient for glucose of from about 5,000 to about 7,000.
The biocompatible membrane of the invention can include a hydrophilic polymer that comprises a polypropylene glycol)-block-poly(ethylene glycol) bis(2-aminopropyl ether). The polysiloxane preferably has a molecular weight of about 500 to about 3,500, and most preferably, about 2,500. In some embodiments, the reaction mixture further comprises a chain extender, such as N-methyl diethanolamine, ethylene diamine, butane diol, diethylene glycol, propane diol or water. The biocompatible membrane of the invention can be the product of a mixture having a glucose diffusion coefficient of from about 1xc3x9710xe2x88x929 cm2/s to about 200xc3x9710xe2x88x929 cm2/s at 37xc2x0 C., or preferably, from about 2.5xc3x9710xe2x88x929 cm2/s to about 10xc3x9710xe2x88x929 cm2/s at 37xc2x0 C.
In a preferred embodiment, the polysiloxane content is from about 15 mole percent to about 75 mole percent of the polymeric portion of the mixture, or more preferably, about 50 mole percent of the polymeric portion of the mixture. In one embodiment, the hydrophilic polymer comprises a combination of a diamino terminated copolymer of polypropylene glycol and polyethylene glycol having an average molecular weight of about 600 and a diamino terminated copolymer of polypropylene glycol and polyethylene glycol having an average molecular weight of about 900. In another embodiment, the polymeric portion of the mixture comprises about 50 mole percent polysiloxane, about 25 mole percent hydrophilic polymer having an average molecular weight of about 600, and about 25 mole percent hydrophilic polymer having an average molecular weight of about 900. Preferably, the hydrophilic polymer comprises a diamino terminated copolymer of polypropylene glycol and polyethylene glycol. A preferred diamino terminated copolymer of polypropylene glycol and polyethylene glycol is poly(propylene glycol)-block-poly(ethylene glycol) bis(2-aminopropyl ether).
The invention further provides an implantable biosensor for measuring an analyte in biological tissue, the biosensor having a coating comprising a biocompatible membrane of the invention. The implantable biosensor can further comprise a transducer that generates a signal upon contact with the analyte. In a preferred embodiment, the analyte is glucose and the transducer is glucose oxidase.
The invention additionally provides a method of measuring an analyte in a tissue of a subject. The method comprises introducing an implantable biosensor of the invention into the tissue of the subject, and detecting the signal generated by the transducer. The amount of signal corresponds to the amount of analyte. Preferably, the analyte is glucose and the transducer is glucose oxidase.